The present disclosure relates generally to methods and systems having temperature transducers or sensors for the measurement of temperatures where high precision is required or in extreme temperature environments such as oil wells. More specifically, some aspects disclosed herein are directed to methods and systems for correcting errors in temperature measurements. The methods and systems provide compensation for errors in temperature measurements due to variations in the measuring instruments that are caused by elevated temperatures.
Resistance Temperature Detectors (RTD) are temperature transducers that utilize, for example, platinum wire resistance elements to measure temperature. One example of such a temperature transducer is shown schematically in FIG. 1. RTDs having thin film resistance elements are also known in the art. As the element becomes hot, the value of the electrical resistance increases. In this, it is possible to correlate the resistance of the element with temperature. Since the element is made from a pure material whose resistance at various temperatures is known, temperature measurements are possible based on a predictable change in resistance of the element as temperature changes.
Typically, the element has a length of wire, such as platinum wire, that is wound around a core of ceramic or glass. Note again FIG. 1. A sheath or pipe of glass, for example, encapsulates the fragile element to form a probe type temperature gauge. Such probes are used for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat.
The lead wires used to connect the RTD to an external display can contribute to measurement error, especially when long lead lengths are involved because of voltage drop across the long lead wires. In particular, such errors are evident in remote temperature measurement locations. It is possible to minimize or limit such errors by the use of 3-wire and 4-wire designs.
Temperature gauges having RTDs that are used for precision measurements of temperature are connected to an instrument to read the resistance of the sensing RTD Rt. Note FIG. 2A. The instrument also measures the amount of current to be injected to the RTD Rt. In this, a reference resistance Rr is located in the instrument to provide reference resistance measurements for purposes of determining the temperature(s) T at which the sensing RTD Rt is located. People normally assume that the temperature of the instrument (more precisely, the temperature of the reference resistance Rr) does not vary, or the variation is very small, in the temperature range of the operation of the instrument.
To measure the resistance of a RTD, the instrument injects current into the RTD. Then, the voltage across the RTD is measured. It is known that current injection into a resistance causes heat dissipation, and the temperature of the RTD may change. A typical resistance of a RTD is 100 ohm. The temperature measurement instrument normally injects 1 mA to the RTD. Such an instrument can also change the injection current, say to 1.4 mA. If the resistance of a RTD measured with a higher current is higher than the resistance that is measured with a normal, i.e., lower, current, it is assumed that the current injection is heating the RTD element. Thus by changing the amount of current it is possible to provide quality control of the temperature measurements.
It is also known that there may be thermo-electric effects (also known as Peltier effects) present in the temperature measurements. The RTD is possibly made of platinum, and the lead wires may be of copper. Any junction of different metallic materials may cause thermo-electricity. The thermo-electricity causes errors in the RTD resistance determination. The temperature measurement instrument is usually capable of changing the polarity of the measurement, i.e., to apply a negative current. By combining two measurements in positive and negative currents, the instrument compensates for the thermo-electric effects.
Temperature gauges utilizing quartz crystal are also known in the art. The natural frequency of a quartz oscillator is a function of temperature. By counting the cycles of oscillation, the temperature of the quartz may be determined. To count the frequency, there should be a time reference. The time reference may be made with another quartz that is insensitive to temperature; however, there is still some temperature dependency. The error may not be negligible if high precision is required, or if the environmental temperature of the reference quartz is high.
In addition to the foregoing, the specifications that typical temperature measurement instruments of the type described herein currently have are accuracy of 0.01 degrees Celsius and resolution of 0.001 degrees Celsius. In certain circumstances, the actual temperature measurement errors as described hereinafter may exceed the instruments' specifications.
In view of the foregoing, applicant recognized a need for improved methods and systems for temperature measurements requiring precision. Specifically, there is need for improved techniques for measuring temperature that compensate for errors that are caused due to temperature effects on the measuring devices. In this, one object of the present disclosure is to provide an improved mechanism for precise measurements of temperature. Another object of the present disclosure is to enable temperature compensated temperature measurements for high precision and/or for extreme temperature applications, such as oil wells. The present disclosure also shows how to compensate for heat dissipation by switching current.